Question

I am trying to do the following mathematical operation with two vectors:

v1 = [a1][a2][a3][a4][a5]
v2 = [b1][b2][b3][b4]b5]

Want to compute:

v = [a2*b2][a3*b3][a4*b4][a5*b5]

Note that I did not want the first element in the new vector.

I was wondering if there is a more efficient (one-liner) way to multiply (element-wise) two vectors in c++ than a for-loop (using push back). My current approach is as follows,

for(long i=1;i < v1.size();++i){
v.push_back(v1[i]*v2[i]);
}

I also tried the following,

 for (long i = 1; i < v1.size(); ++i){
     v[i-1] = v1[i]*v2[i];
 }

Any suggestions?

Answer 1

std::transform( v1.begin()+1, v1.end(),
                v2.begin()+1, v.begin(),  // assumes v1,v2 of same size > 1, 
                                          //       v one element smaller
                std::multiplies<int>() ); // assumes values are 'int'

You can replace v.begin() with std::back_inserter(v) if v is empty, you should reserve() memory upfront to avoid multiple allocations.